1. Field of the Invention
This invention relates generally to borehole logging apparatus and methods for performing measurement while drilling applications. More particularly, this invention relates to a new and improved apparatus and method for effecting determination of toolface orientation using downhole magnetometers.
2. Background of the Art
Oil well logging has been known for many years and provides an oil and gas well driller with information about the particular earth formation being drilled. In conventional oil well logging, after a well has been drilled, a probe known as a sonde is lowered into the borehole and used to determine some characteristic of the formations which the well has traversed. The probe is typically a hermetically sealed steel cylinder which hangs at the end of a long cable which gives mechanical support to the sonde and provides power to the instrumentation inside the sonde. The cable also provides communication channels for sending information up to the surface. It thus becomes possible to measure some parameter of the earth's formations as a function of depth, that is, while the sonde is being pulled uphole. Such “wireline” measurements are normally done in real time (however, these measurements are taken long after the actual drilling has taken place).
Measurement-while-drilling logging either partly or totally eliminates the necessity of interrupting the drilling operation to remove the drillstring from the hole in order to make the necessary measurements by wireline techniques. In addition to the ability to log the characteristics of the formation through which the drill bit is passing, this information on a real time basis provides substantial safety advantages for the drilling operation.
One potential problem with MWD logging tools is that the measurements are typically made while the tool is rotating. Since the measurements are made shortly after the drillbit has drilled the borehole, washouts are less of a problem than in wireline logging. Nevertheless, there can be some variations in the spacing between the logging tool and the borehole wall (“standoff”) with azimuth. Nuclear measurements are particularly degraded by large standoffs due to the scattering produced by borehole fluids between the tool and the formation.
There are several teachings in prior art that involve partitioning a cross-section of the borehole into a number of sectors. For example, U.S. Pat. No. 5,397,893 to Minette, teaches a method for analyzing data from a measurement-while-drilling (MWD) formation evaluation logging tool which compensates for rotation of the logging tool (along with the rest of the drillstring) during measurement periods. U.S. Pat. No. 5,513,528 to Holenka et al teaches a method and apparatus for measuring formation characteristics as a function of azimuth about the borehole. The measurement apparatus includes a logging while drilling tool which turns in the borehole while drilling. The down vector of the tool is derived first by determining an angle φ between a vector to the earth's north magnetic pole, as referenced to the cross sectional plane of a measuring while drilling (MWD) tool and a gravity down vector as referenced in said plane. The logging while drilling (LWD) tool includes magnetometers and accelerometers placed orthogonally in a cross-sectional plane. Using the magnetometers and/or accelerometer measurements, the toolface angle can usually be determined. The angle φ is transmitted to the logging while drilling tool thereby allowing a continuous determination of the gravity down position in the logging while drilling tool. Quadrants, that is, angular distance segments, are measured from the down vector. U.S. Pat. No. 6,584,837 to Kurkoski having the same assignee as the present application and the contents of which are fully incorporated herein by reference teaches a method of azimuthal and offset binning for analysis of nuclear data in an MWD environment.
Minette, Holenka and Kurkoski do not address possible sources of error in relying on magnetometer readings made using magnetometers on a rotating drillstring. One source of error is the nonuniform rotation speed of the drillstring. A second problem is the time delay inherent in the electronics. Measurements may be made simultaneously by the formation sensor and the orientation sensors, but there is a time delay between the time the measurements are made with the two types of sensors and the time at which they are processed. The interaction between the two sources of error, i.e., nonuniform rotation and time delay, can be fairly complex as discussed below. The problem of nonuniform rotation is partially addressed in copending U.S. patent application Ser. No. 10/629,268 of Cairns et al. having the same assignee and the contents of which are fully incorporated herein by reference. However, addressing the non-uniform rotation by itself gives only a partial solution. In addition, there is the problem of bias in the orientation sensor measurements. Generally, magnetometers are preferred as orientation sensors over gyroscopes, and magnetometers are susceptible to errors causes by metallic drill collars, casing, and accumulated debris. There is a need for a method of determining accurate orientation values using measurements made by a magnetometer on a MWD logging tool. The present invention satisfies this need.